1. Field of the Invention
The present invention relates to an optical disc for storing information by irradiating laser light onto an optical disc medium, and an optical disc device for same.
2. Description of the Related Art
In recent years, great developments have been made in optical disc devices as means for storing and reproducing large volumes of data, and approaches for achieving high storage density have been adopted, one of these methods being a phase change-type optical disc device utilizing reversible crystal--non-crystal state changes.
In a phase change-type optical disc device, marks (amorphous sections) and spaces (crystalline sections) between these marks are formed on an optical disc medium, by irradiating semiconductor laser light onto the optical disc medium using two power settings: a peak power for rendering crystalline regions amorphous, and a bias power for crystallizing amorphous regions.
Land/groove recording technology exists for recording these marks and spaces in tracks in both the land region and the groove region of a guide groove on a disc.
In order to improve the reliability of optical discs, it is necessary to store and reproduce high-quality signals on the optical disc. If there is an inclination (tilt angle) of the recording surface of an optical disc with respect to the optical axis of the light beam, then an aberration occurs in the light spot and it becomes difficult to record and reproduce high-quality signals on the optical disc. Therefore, in order to record and reproduce signals to and from the optical disc, it is necessary to detect the aforementioned tilt angle accurately, and to correct this tilt angle.
A conventional method for correcting tilt by a tilt device is shown in FIG. 2.
In FIG. 2, 201 is an optical disc, 202 is an optical head which focuses a light beam onto the optical disc, 203 is a tilt platform, 204 is a signal calculating circuit, 205 denotes focus control means for controlling the focal position of the light spot on the optical disc surface, 206 denotes tracking control means for controlling the position of the optical spot on the track, 207 is a tilt sensor for irradiating light for detecting the inclination of the optical disc signal with respect to the optical axis of the aforementioned optical beam, onto the optical disc, receiving light reflected by the optical disc, and detecting the inclination of the recording surface of the optical disc with respect to the optical axis of the aforementioned light beam, and 208 denotes tilt control means for controlling the inclination of the recording surface of the aforementioned optical disc with respect to the optical axis of the aforementioned light beam, by inclining the aforementioned tilt platform according to the value detected by the aforementioned tilt sensor.
FIG. 3 shows a graph of a case where the tilt position is corrected at both the inner circumference and outer circumference of the optical disc, in a conventional optical disc device.